The present invention relates generally to battery technology, and more particularly to battery cells that incorporate features for sensing in-situ characteristics and parameters of the battery cell.
Battery cells are implemented in various applications, such as those applications required for vehicle powertrain designs that rely on electrical traction power, e.g., hybrid-drive, and electric-drive vehicles. Such applications generally require that the battery cell not only output power on the scale of about 3V to 5V, but also retain favorable performance characteristics such as longer cycle life, increased robustness, and improved reliability. These demands are usually met by connecting arrays of battery cells together in series and/or parallel configurations to form larger, more complex battery packs. The design of battery packs, however, is difficult because of the delicate balance that is to be struck between maximizing efficiency and reliability, while retaining the necessary power output, and other performance characteristics of the battery cell, the battery pack, and the overall drive system. On the other hand, it is recognized that if the characteristics and the parameters of the battery cell were better understood (e.g., as through monitoring), then the battery cell could operate closer to its maximum limit. Such maximum limit could, in effect, be greater than the required scale mentioned above, but indeed greater than about 6V.
It has been found that to meet these design challenges there are benefits to providing battery cells of larger sizes. These advantages include a reduction in the number of cells that are needed to meet the target output power for a battery pack. A reduction in number of cells, in turn, minimizes cell-to-cell variations in the battery pack (by requiring less cells), and reduces the peripheral circuitry and electronics that is necessary to operate the battery pack. The end result is a reduction in the complexity that leads to lower system cost and higher reliability.
But while the larger cells may meet certain specific design criteria, incorporating cells of larger sizes into battery packs creates several design and functional challenges. These challenges include the uncontrolled heat generation that can occur in the event of cell failure. The challenges also include the lack of an efficient battery cell design that maximizes power output, while also maintaining reasonable operating temperatures.
These issues are particularly important because the performance characteristics of battery cells, e.g., prismatic battery cells, and related technology are very sensitive to characteristics of the physical processes that take place inside of the cell structure. That is, changes in one or more particular aspects of the cell can not only be detrimental to the performance of the cell, but also to the performance of the battery pack that incorporate numerous battery cells in a single package. For example, cell temperature is an indicator of the electrolytic reaction occurring within the ion exchange membrane in the battery cell. During high stress conditions, such as during aggressive charge and discharge events, the battery cell can generate excessive heat. This heat can deteriorate the battery cell, and in some cases result in thermal runaway, a condition in which an irreversible reaction occurs that results in the failure of the polymer ion exchange membrane. Thermal runaway and related conditions can also cause catastrophic failure of the individual cells. This failure is not only detrimental to the cell, but also to the battery pack that incorporates numerous cells in series/parallel configuration because the failure of one cell can cause an open-circuit condition across a series array of battery cells, as well as the deterioration of the performance of the battery pack.
Unfortunately the construction of battery cells is sensitive to local changes in geometry, as well as to localized forces that act on the structures of the battery cell. This sensitivity is such that it prevents many sensing devices (e.g., thermocouples, thermisters, and strain gages) from being implemented in and around the battery cells because these sensing devices have three-dimensional bodies and wiring that can cause localized deformation of the thin cell wall that are typically used to construct the battery cells. Moreover, since the battery packs are often being optimized for size, there is typically very little space between the battery cells for any type of sensing structure, let alone those that have expansive three-dimensional characteristics. Thus, in order to gather any information about the operating characteristics of the battery cells without disrupting its overall performance, discrete three-dimensional sensors can only be located around the edges of the battery cells. And due to packaging constraints at the battery pack level of construction, the sensors can only be placed at staggered distances throughout the outer periphery of the battery pack. For example, the constraints limit the available number of sensors to one (1) for every four (4) to eight (8) battery cells in the battery pack.
Due to the above constraints, the information that is gathered from the current generation of sensors is limited, slow, and in some cases inaccurate and misleading. For example, positioning temperature sensors on the exterior edges of the cells leads to inaccurate measurement of the cell temperature because the temperature at the edge of the cell is affected by ambient conditions that exist around the battery pack. The location of the sensor can cause measurements of the temperature at the edge to lag behind the actual temperature at the critical portions within the battery cell. Such poor correlation between the measured temperature and the actual temperature can compromise the battery pack because it is not a true reflection of the operating characteristics of the battery cells, and thus an inaccurate representation of the battery pack generally. Therefore, a better solution is to provide map of the temperature of the entire battery cell within the pack structure so as to minimize the thermal resistance between the sensor and the area within the battery structure
Another limitation is the type of measurements that can be made using the current generation of sensors. As mentioned above, physical constraints generally restrict these sensors to locations at the periphery of the battery pack. So operating conditions such as pressure, ionic flow, and electrochemical processes can not be measured because these conditions require access to the interior of the cell.
Therefore, there is a need for a battery cell with improved operational characteristics. It is also desirable that this battery cell comprises, or is compatible with sensing, monitoring, and data gathering devices so as to provide and enrich the understanding of the operating parameters of the battery cell, the battery pack, and the overall drive system.